Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Milwaukee/Sullivan WI
306 PM CDT Monday Sep 1 2014
Tonight - confidence...medium
surface cold front combined with strong and broadly cyclonic middle/upper
flow and limited instability...around 500 j/kg MLCAPE...will maintain scattered
rain showers/isolated thunderstorms and rain into the early evening hours. Mesoscale models have a pretty
good handle on these trends so did not deviate much in this regard. Then
expect northwest winds to usher in drier airmass as the night wears along...per
idsodrosotherm gradient sagging southeast across northwest WI/southeast Minnesota.
Tuesday - confidence...medium
another shortwave is expected to track through WI through early afternoon.
Models have boosted quantitative precipitation forecast a bit from the prior run. Airmass will be
considerably drier and while expecting some cape to evolve there will be
quite a bit of dry air in the column as well. Upper jet structure not
placing southern WI in a favorable quadrant for forcing either. Will keep rain showers
chances in mainly the northern County Warning Area maintaining a model blend in mind. 925
temperatures 18-20c so looking for highs in the middle 70s to around 80.
Tuesday night and Wednesday...forecast confidence is high
A zonal upper level flow extends across the U.S./Canadian border
area as a trough moves across the northwest U.S. Weak upper
divergence pushes into southern Wisconsin Wednesday afternoon.
Weak 850/700 mb warm air advection begins...then increases especially
at 850 mb Wednesday afternoon. Moist 850 mb dew points of 15 celsius
arrive over south central Wisconsin during the afternoon as 850 mb
winds begin to increase. 700 mb upward motion is weak.
Weak high pressure moves across the upper Ohio Valley and Great
Lakes early Wednesday. The 12z GFS and 00z European model (ecmwf) are dry across
southern Wisconsin...but the NAM does bring in light precipitation
into portions of south central Wisconsin Wednesday afternoon.
Wednesday night and Thursday...forecast confidence is high
A weak ridge tries to briefly build into Wisconsin as the
middle/upper shortwave approaches North Dakota and southern Manitoba.
The weak upper divergence lifts northeast across southern
Wisconsin Wednesday night. The 850 mb low level jet punches
northeast into southern Wisconsin Wednesday evening...increasing
to 40 knots by midnight before weakening late. 850 mb dew points
rise to 16 to 18 celsius over much of southern Wisconsin. Elevated
cape increases to 1300 joules/kg if lifted from just above the
shallow low level inversion. 700 mb upward motion is weak...but
does increase late...especially north of Milwaukee.
A warm front and precipitation moves across southern Wisconsin
Wednesday night on the 12z NAM/GFS and 00z European model (ecmwf) and into
Thursday morning on the GFS. Then the bulk of the showers and
thunderstorms are to the north Thursday afternoon with southern
Wisconsin in the humid warm sector. Mixed zero to 1 km cape
values are around 3500 joules/kg Thursday afternoon with a cap of
around 60 to 100 joules/kg around 850 mb in the southeast...that
weakens to around 15 joules/kg by evening in areas northwest
toward The Dells.
Thursday night and Friday...forecast confidence is high
The GFS and European model (ecmwf) have a southwest flow across Wisconsin as a
shortwave moves across south central Canada and the upper Great
Lakes. Meanwhile a large ridge extends across the southern U.S.
Both the 12z GFS and 00z European model (ecmwf) push a cold front across most of
southern Wisconsin Thursday night and Friday morning. The GFS has
heavier precipitation than the 00z European model (ecmwf) and lingers it longer.
Friday night and Saturday...forecast confidence is high
The middle/upper trough heads into southeast Canada with a northwest
flow over Wisconsin on Saturday.
Both models push the surface front to the south...reaching the
Ohio River by Saturday. The GFS still lingers moderate amounts of
rain Friday evening while the 00z European model (ecmwf) is mainly dry.
High pressure builds into Wisconsin from the northern and Central
Plains on Saturday.
Sunday and Monday...forecast confidence is medium
The flow becomes more zonal Sunday and Monday. The GFS has a
stronger west trough with more of a southwest flow Monday than the
00z European model (ecmwf).
A large surface high is expected to extend across the Great Lakes
region Sunday. This high still extends back into Wisconsin on the
00z European model (ecmwf)...but recedes more on the 12z GFS Monday as precipitation
reaches south central Wisconsin late.
Low pressure lifting northeast from Lake
Superior with trailing cold front generating rain showers/isolated thunderstorms and rain. These
will likely move across southern WI into the early evening hours. After frontal passage dry
air advection and loss of heating will decrease cumulus coverage. Weak
high builds in tonight with surface trough possibly generating a few rain showers
Tuesday afternoon though expect these mostly north of taf sites.
Marine...extended Small Craft Advisory into the early evening. Tight
pressure gradient will allow for gusty southwest winds to 25 knots.
Winds and waves will subside towards middle evening.
Lm...Small Craft Advisory until 9 PM CDT this evening for lmz643>646.
Tonight/Tuesday and aviation/marine...collar
Tuesday night through Monday...hentz